Compositions and method for complex coatings



United States Patent Oflice 3,333,249 Patented May 14, 1958 3,383,249 CGR'IPOSITIGNS AND METHOD FOR COMPLEX COATINGS Edward Heinzelman, .ln, Palisades Park, N..l., assignor to Oakite Products, Inc., New York, N.Y., a corporation of New York No Drawing. Continuation-impart of application Ser. No. 153,717, Nov. 20, 1961. This application Mar. 15, 1967, Ser. No. 623,216

18 Claims. (Cl. 148-62) ABSTRACT OF THE DISCLOSURE This invention embraces water-soluble mixture compositions and water-dilutable aqueous concentrates thereof, which contain as their essential ingredients (a) a chromium trioxide-providing substance and (b) a Watersoluble lithium-cation-providing constituent containing (i) a minor portion of a lithium halide other than its chloride and (ii) a major portion of at least one more readily soluble lithium salt; from which compositions or concentrates there are prepared dilute aqueous conversion or complex coating baths, in which are carried out the method of the invention wherein ferrous and nonferrous metals subjected to immersion in the baths receive over their immersed surfaces a corrosion resistant reaction conversion coating.

This application is a continuation-in-part of my copending application Ser. No. 153,717, filed Nov. 20, 1961, now abandoned.

This invention is that of water-soluble mixture compositions and water-dilutable aqueous concentrates thereof, which contain as their essential ingredients (a) a chromium trioxide-providing substance and (b) a water-soluble lithium-cation-providing constituent containing (i) a minor portion of a lithium halide other than its chloride and (ii) a major portion of at least one more readily soluble lithium salt; from which compositions or concentrates there are prepared dilute aqueous conversion or complex coating or treating baths. These baths, as a part of the invention, produce highly corrosion resistant reaction or complex or conversion coatings on the surfaces of oxidation corrosion prone ferrous and also non-ferrous metals immersed in these coating bath solutions. (Immersion includes wetting by submerging, dipping in, brushing on, spraying, rolling, or otherwise drenching).

The invention includes also the method of producing such complex or conversion coatings on such metal surfaces by (i) immersing objects or devices of any of those various metals in an aqueous coating bath containing (a) the chromic acid (i.e. chromium trioxide or chromic anhydride) and (b) the mixed water-soluble lithiumcation-providing constituent salts, and with the ingredients (a) and (b) being in such ratio to one another and sufficient respective concentration in the coating bath to produce such coating on the immersed surfaces of the treated product, and (ii) in a time sufficient for the desired extent of conversion coating to be obtained.

A feature of the invention is that not only the dry mix compositions, but also the aqueous concentrates as well as the aqueous coating baths prepared from either of them are entirely stable and have unusually extended life.

This feature of unusual stability of these various composition forms of the invention is shown by the aqueous concentrates, and especially by the diluted coating baths, by their giving no or at most insignificantly little sedimentation, even at elevated temperatures at which they generally are used. These compositions thus are dependably reproducible.

A further feature of the compositions of the invention is that the coating baths serve readily to produce reaction, complex or conversion coatings without any special pretreatment (e.g. acid etch) of the metal surface other than any suitable one of the common cleaning treatments when the metal surface to be treated initially is soiled so as to need cleaning. Thus, the method of the invention is simple and remarkably practical.

Yet another feature of these compositions is their applicability to both ferrous and non-ferrous metal surfaces.

A still further feature of these compositions is their utility to provide a beneficial conversion coating applicable not only as a finished coating but also as a corrosion resistant base or under-coating for a subsequent organic outer coating such as an organic resin, lacquer or paint. These conversion coatings thus manifest improved adhesion and render superior service. When used as the finished coating, the conversion coating produced provides superior corrosion resistance without any organic or other supplemental finish.

When the conversion coatings to be produced with the compositions and methods of this invention are to serve as final coatings, the initial dry mix composition or aqueous concentrate and treating bath can include a water-soluble or water-dispersible dye or pigment compatible with chromic acid to provide the corresponding color in the finished conversion coating.

An additional feature of the invention is that the complex or conversion coatings provided are resistant to organic solvents and manifest exceptional adhesion and ductility and corrosion resistance.

Still a further feature of the invention is the ease and quickness with which the complex or conversion coating is produced by its compositions, baths and method, generally in a comparative short time of exposure to the bath such as within about thirty seconds and as little as several or a few seconds. The articles treated in the conversion coating baths of this invention after Withdrawal from them need no special further treatment other than simple drying usually at a suitable temperature, and readily simply with hot air, or merely a water rinse followed by such drying.

The color of the finished metallic complex coating generally does not change when, after having been dried, the article then is heated to a temperature above that at which it was dried; except as to any organic dye in the coating when heated to a temperature at which the dye can discolor, or to any pigment in it, when to a higher temperature at which its color might change.

Consider broadly, the compositions of the invention, whether dry mix, aqueous concentrate, or diluted treating bath, contain (a) chromic acid and/or water-soluble dichromate together with (b) a water-soluble lithiumcation-providing component containing (i) a minor portion of a lithium halide other than its chloride, and (ii) a major portion of at least one more readily water-soluble lithium salt with (a) and (b) being so proportioned that the chromium trioxide content markedly exceeds the total content of lithium cation of its salts.

The number of different metal surfaces on which the bath composi'ions of the invention produce highly beneficial corrosion resi tant conversion coatings as Well as the number of different applicable water-soluble lithium sats, and the Widely effective pH range, make it diflicult to give a single numerical range of content or proportions for the chromium and lithium components required as the essential ingredients. to fit all of the possible combinations of the several variables.

Ordinarily, chromium by weight should be at least equal to, and beneficially greater than, that of total lithium cation. A good practical minimum is about 1.5 parts of chromium to one part of lithium, and better yet at least about five parts of chromium to about three parts of lithium. For generally good practical operation, the chrotium should be at least about seven times the lithium cation content (i.e. in both portions of the water-soluble lithium salts) and can extend to about one thousand parts of chromic oxide per part of lithium cation.

The aqueous concentrate can contain the substance providing the chromic oxide (i.e. whatever its source) in a weight range of from about one to ninety parts of chromic oxide to from about three-tenths, or even as little as one-tenth, part of lithium cation.

The dilute treating bath then can contain the chromic acid (of whatever source) and the lithium cation within the various ratios of chromium to lithium indicated in the two immediately preceding paragraphs. However, such bath should contain by weight a minimum of about three-tenths percent of lithium cation (of its water-soluble salts) with the corresponding amount of chromic oxide content within the ratio ranges therein indicated. The generally practical minimum concentration of chromium is about five-tenths percent.

The chromium trioxide (i.e. chromic anhydride or chromic acid) can be used as such in any of its commercially readily available forms. The readily available technical grade of its granular powder form is most desirable and beneficial. However, chromium trioxide can be replaced in part or as a whole, for example, by the chromium trioxide equivalent of any water-soluble dichromate such as an alkali metal dichromate such as sodium dichromate, potassium dichromate, lithium dichromate, and ammonium dichromate (considered as if it were an alkali metal dichromate), or any compatible water-soluble polyvalent metal dichromate such as zinc dichromate.

As the lithium halide other than its chloride, so far as presently indicated, lithium fluoride appears to be more effective than lithium bromide or iodide.

For providing the major portion of lithium cation, any different water-soluble lithium salt with either an inorganic or organic anion can be used, such as lithium nitrate, lithium chloride, lithium sulfate, lithium chromate, lithium dichromate, lithium molybdate, lithium titanate, lithium thiocyanate, and lithium vanadate; or that of a lower alkanoic acid, as lithium formate and lithium acetate, or lithium valerate, or of a lower dicarboxylic acid as lithium succinate, or of such hydroxy acid, e.g. lithium citrate, lithium lactate, lithium oxalate, lithium tartrate, and lithium bitartrate; or that of benzene substituted with an acid group, as lithium phenolate from a hydroxybenzene, lithium benzoate from benzoic acid, lithium phenolsulfonate from a hydroxy benzenesulfonic acid, lithium salicylate from a hydroxy benzoic acid, and lithium acetylsalicylate, and others.

Any water-soluble lithium salt of an organic acid which is inert to chromic acid is applicable (although in some operations some of them may not be readily desirable because of cost). Moreover, while more than one such lithium salt, with one in a minor portion being a halide other than its chloride, must be used in the same composition, it is possible to use as a non-halide salt lithium dichromate without separate chromic acid because lithium dichromate furnishes both chromic acid (in the form of dichromate) and lithium cation within the applicable ratios.

The lithium halide (other than chloride) portion of the lithium compounds essential constituent of the compositions of the invention is present therein in such proportion to the different lithium compound that the lithium content of acid (non-chloride) lithium halide is from about 0.7% to about of lithium in the lithium nitrate equivalent of said different lithium compound.

The active highly corrosion resistant conversion coating producing co-acting constituents of the compositions of the invention consist essentially of (a) the chromium trioxide (or water-soluble dichromate) and (b) the two different essential accompanying water-soluble lithium salts. However, any embraced dry mix, aqueous concentrate, or treating bath can contain also a minor percentage of one or more compatible agents for any other specific purposes.

Such agent can be a buffering agent, for example, an alkali metal acetate as sodium or potassium acetate (e.g. for pH 2) or ammonium acetate, or an alkali metal phosphate such as sodium or potassium dihydrogen phosphate, or ammonium dihydrogen phosphate included with them, or the citrates of those same cations, or sodium tetraborate, potassium hydrogen tartrate, or potassium hydrogen phthalate, to serve to stabilize the resulting treating bath at a desired pH.

In some circumstances, a compatible surface active agent, beneficially an anionic one, such as sodium lauryl sulfate, mixed sodium lauryl and oleyl sulfates, or amine mixed long chain (i.e. lauryl and oleyl) sulfates, can be included to enhance certain operating steps, for example, drainage of adhering or entrained bath solution from the treated articles being removed from the bath, or to enhance the subsequent rinsing step. Advantageously effective are the perfiuoroalkyl sulfonic acids anionic surfactants having eight carbon atoms (with alkyl being generic to open chain alkyl as well as dimethyl and ethyl substituted cyclohexyl) by use to the extent of, say, from about 0.1 to about 0.5 percent by Weight of a concentrate.

The compositions of the invention serve to produce the highly corrosion resistant conversion coatings on a metal ordinarily subject to oxidation corrosion on sufficient exposure therefor to an ambient oxidation corrosiondnducing atmosphere. Examples of such metals are steel, wrought or cast iron and other ferrous metals. Other such oxidation corrosion susceptible metals which can be correspondingly similarly highly corrosion resistant conversion coated are non-ferrous metals not only such as the so-called white metals aluminum, magnesium, zinc, and their alloys (not alone those wherein each of them separately predominates), but also nickel, titanium, cadmium, copper, and tin, and alloys such as of the latter two, as brass and bronze and others.

The conversion coatings produced by the treating baths of the invention, for example, prevent so-called white rust on non-ferrous metals such as aluminum and zinc.

Speaking generally, the method of the invention comprises immersing an article or device of the specific ferrous or non-ferrous metal, or composed of parts of different ones of those metals, in an aqueous treating bath containing an adequate amount of both the lithium halide other than its chloride and the other different watersoluble lithium salt, to provide a sufficient concentration of lithium cation, together with the corresponding quantity of chromic acid (whatever its source) at least about double the quantity of lithium cation, and for a time sufficient to produce a suitably highly corrosion resistant conversion coating under the operating conditions; removing the article or device when development of the desired coating on it is noted. If the resulting conversion coating is to be a final coat, the device then is dried. However, if it is to serve as an undercoat for an organic finish coat such as a lacquer or paint, it can be rinsed and, if necessary, dried before applying the finish coat. The bath should contain by weight at least about one percent of chromium trioxide and about one-tenth, and beneficially at least about three-tenths, percent of lithium cation.

The various types of compositions, as well as the method of this invention are illustrated by, but not restricted to, the following examples:

DRY MIXES Example l.Chromic oxide, lithium fluoride and other lithium salt alone: 75.8 pounds of chromic oxide granular powder were admixed uniformly with 21 pounds of lithium nitrate, and 3.2 pounds of lithium fluoride.

Example 2.With bufier: The respectively same amounts of the three ingredients of Example 1 were admixed with 3.1 pounds of potassium acetate.

Example 3.-With wetting agent: The respectively same amounts of the three ingredients of Example 1 were admixed with 0.2 pound of perfiuoro normal-octyl sulfonic acid.

AQUEOUS CONCENTRATES Example 4.Chromic acid, lithium fluoride, and lithium nitrate, and water: 62 pounds of the dry mix of Example 1 were dissolved in 38 pounds of water and the resulting solution stirred to provide uniformity of concentration.

Example 5.With bufiering agent: 62 pounds of the dry mix of Example 2 were dissolved in 38 pounds of water, and the resulting solution stirred to make the solution uniform.

Example 6.With wetting agent: 62 pounds of the dry mix of Example 3 were dissolved in 38 pounds of water, and the resulting solution was stirred to make the solution uniform.

The lithium fluoride of any of the Examples 1 through 6 can be replaced in part or as a whole by the respectively corresponding lithium equivalent amount of lithium bromide or lithium iodide. So also the chromic oxide (CrO of any of those examples as well as of any herein described modifications thereof, depending on cost and other considerations, can be replaced at least in part or as a whole by about the corresponding chromic oxide equivalent amount of any water-soluble dichromate such as any of those earlier above mentioned.

Then too, the water-soluble different second or other lithium salt of any of the examples, and of any of the herein described modifications of any of them can be replaced in part or as a whole by about the corresponding lithium cation equivalent of any other water-soluble lithium salt so long as its anion is not a halogen other than chlorine and is inert to chromic acid regardless of its source at the selected operating conditions.

The buffering agent of the dry mix of Example 2 can be used within the range of from about 2 to about 4 percent. It can be replaced in both Examples 2 and 4 and any modifications of either of them by any other compatible buffering agent of those earlier above described or named. Also, the wetting agent of the dry mix of Example 3 can be used within the limits of from about 0.1% to about 0.5%. It likewise can be replaced in either Example 3, 6 or 7 or any modification of any of them, by any other perfluoro-octyl (whether open chain or dimethylor ethyl-substituted cyclohexane) sulfonic acid such as perfiuoro 2,5-dimethycyclohexyl sulfonic acid and 4-ethylcyclohexyl sulfonic acid.

Any composition containing a wetting agent and any containing a buffering agent is not restricted to the one added agent alone. Any composition can contain both such agents, as shown but not limited to:

Example 7.Dry mix with bulfering and wetting agent: The same respective amounts of the separate ingredients of the dry mix of Example 2 and 0.2 pound of perfluoro normal-octyl sulfonic acid are intimately mixed together, for example, by first admixing the small amounts of the lithium fluoride and the buffering and wetting agents; then intimately mixing their mixture with the lithium nitrate, and then mixing the resultant admixture with the chromic oxide, thereby providing a uniform dry mix of all of the five ingredients.

The perfluoro normal-octyl sulfonic acid can be replaced by a similar amount of any other perfiuoro-octyl sulfonic acid or by from about one-fifth pound to about 1.5 pounds of any other compatible anionic or even nonionic wetting agent inert to chromic acid.

A suitable concentrate of the dry mix of any of Examples 1, 2, 3 or 7, or any modification of any of them, can be prepared by dissolving the mix in at least sufficient water to dissolve it completely and enable it to remain in solution in the ambient temperature of the area where it is to be used. Generally it is better to use a practical excess over that, and advantageously sufiicient to enable the concentrate to be completely liquid enough for it to be dispensed by automatic control. Ordinarily, from about to about water in the concentrate can be sufficient for most applications although for some uses it may be desirable to include still more, say, up to about or or even about of water. The concentrate is further illustrated by, but not to be limited to, the following:

Example 8.Concentrate having both buffering and wetting agents: 57 pounds of the dry mix of Example 7 are admixed into 43 pounds of water and their mixture stirred until solution is complete and uniform.

None of the ingredients of any of the complete examples or any of the herein described modifications of any of them need be restricted to the specific respective amounts of their individual ingredients. Any of them can be varied within the repectively elsewhere herein given limits and relative proportions for it and depending on the metal to be treated and shape of the device and other conditions.

In order to avoid making this specification prolix, each of the different examples possible from all of these herein so described various substitutions into any of them is considered as though it were completely described as if each of them actually appeared written out in full herein.

Conversion coating baths are prepared by dissolving from about 1 to about 5 parts by weight of any of the foregoing concentrates in from about to about 99 parts of water, or alternatively by diluting from one-half pound to ten pounds of any of these concentrates respectively with an amount of water sufficient to make a total of one hundred pounds of the resulting dilute treating bath composition.

CONVERSION COATING BATHS AND METHOD In the following examples panels of cold rolled low carbon steel and also of SAE 3003-H14 aluminum were treated as recited in the respective examples:

Example 9.-(i) The steel panels first were cleaned by immersion for five minutes in an alkaline cleaning bath maintained at F. to 200 F. and containing one pound per gallon of water of an alkali and chelate cleaner dry mix composed of 62.5% of caustic soda, 31.25% of sodium glucona-te, and 6.25% of a non-ionic wetting agent.

The aluminum panels initially were cleaned by immersion for five minutes in a silicated alkaline cleaning bath maintained at 180 to 200 F. and containing from four to eight ounces per gallon of water of the anhydrous sodium metasilicate, sodium bicarbonate, sodium tripolyphosphate, and wetting agents cleaning composition shown in Metal Progress of November 1959, page 114, Alkaline Cleaning of Metals, Table IV.

(ii) The panels then were put through a cold water rinse usually for a half minute to a minute.

(iii) Then the thus rinsed panels were immersed for thirty seconds in an aqueous conversion coating bath solution containing 2 parts by weight of the aqueous concentrate of Example 1 dissolved in 98 parts of water.

(iv) The panels then were removed and blown dry by a hot air blast.

(v) The steps of this example were repeated on panels of the same two metals, "but instead of drying them by hot air blast in its step (iv), the panels were baked in an oven for five minutes at 250 F.

In each case the color of the conversion coating of the aluminum panels had an iridescent to golden brown, and that of the steel panels was a bluish brown to brown.

The various steps of the two foregoing modifications of this example were repeated, but instead of immersing the panels in the step (iii) conversion coating bath, the rinsed panels were immersed in a dilute treating solution containing by weight 2 parts of the concentrate of Example 6 dissolved in 98 parts of water. The hot air dried as well as the baked aluminum and steel panels showed respectively about the same colors as after treatment in the bath of Example 9, but manifested improved uniformity over the entire coating.

Example 10.-Dyestufi colored: Steel panels as in Example 9 were cleaned with the alkali and chelate cleaner of its step (i) and rinsed as in its step (ii).

The rinsed panels then were immersed for one to two minutes at ambient temperature in a conversion coating bath composed of five parts by volume of the concentrate of Example 4 diluted with ninety-five parts by volume of water, .to which bath was added one-half percent by weight of the water-soluble dye National PD & C Red #4Ponceau SX (available from National Aniline & Chemical Co.).

The panels then were removed and dried at 150 F. for five minutes. The complex conversion coating was red and manifested corrosion resistance (tested by salt spray) equal to and even somewhat better than that of the nondyed coating produced by a conversion coating bath containing about the same ratio of chromic oxide and lithium; cation-providing compounds.

In place of the specific dye used in this example, any other suitable water-soluble, acid stable dye can be used.

The pH of the complex conversion coating baths containing dyes was adjusted with sodium hydroxide solution in respective runs from about 2.0 to 6.4 to note the effect of pH on absorption of dye into the resulting coating. Improved coatings (as to absorption of dye) was noted at the higher pH level (i.e. 6.4). This indicates the benefit of taking into consideration the optimum pH level recommended for a particular dye.

Example 11.--Pigment incorporated: Steel panels as used in Example 10 were cleaned and rinsed as described therein. The rinsed panels then were immersed for one to two minutes at ambient temperature in a treating bath composed as in Example 9 but into which there was admixed parts by weight of carbon black.

The panels then were removed and dried at 150 F. for five minutes. They were coated with a hard even black coating which manifested corrosion resistance (to salt spray) equal to and even somewhat better than that of the non-pigmented coating produced *by a conversion coating bath of substantially the same composition without the carbon black.

The carbon black of the conversion coating bath of this example can be replaced *by the equivalent concentration of any other compatible pigment, for example, black iron oxide, red iron oxide, chrome green, and others. The various steps of this example are to be considered as repeated separately respectively with the carbon black of the conversion coating bath replaced by an equal amount of each of these other specific pigments respectively, with the resulting coating obtained showing the respective color of the particular pigment and the same quality of corrosion resistance.

Use of reaction or conversion coating baths of the invention is not restricted merely to aluminum, steel, and galvanized steel. Suitably effective corrosion resistant reaction or conversion coatings can be obtained by relatively similarly treating a ferrous or non-ferrous metal ordinarily subject to oxidation corrosion after being sufiiciently exposed to an ambient oxidation-corrosion-inducing atmosphere, such as are much earlier above described.

The bath temperature used may be up to about 200 F. with a better range from about 80 to about 120 F., and optimally from about 90 to 100 F. The choice can be governed by the particular conditions met and results desired.

The conversion coating treating baths and the method of the invention are not limited to any particular pH. In general, the baths of the various examples, especially those prepared with chromic oxide, fall within the range of from about pH 1 to about pH 2. Also, satisfactory and suitable coatings were, and are, obtained with baths up to pH 6 and even with baths adjusted with an alkali such as sodium hydroxide to a pH beyond that, as a generally good upper limit, with good practical operating even up to pH 10.

The examples wherein chromic oxide is the sole chromic acid source need not be limited to that alone. Depending on cost and other considerations, the chromic acid can be replaced at least in part by about the chromic oxide equivalent amount of any water-soluble dichromate such as any of those referred to above.

The water-soluble lithium salt of any of the examples can be replaced in part or as a whole by about the lithium cation equivalent of any other water-soluble lithium salt so long as its anion is other than fluorine and is inert to chromic acid (whether from chromic oxide or a watersoluble dichromate) at the selected operating conditions.

The various mixtures, concentrates and conversion coating treating baths of all of the examples show that the compositions and method of the invention depend on (i) the chromium trioxide providing constituent and (ii) the two lithium-cation-providing constituents as the required co-acting essential conversion-coating-producing constituents to provide the benefits of the various compositions and the method of the invention.

For that reason, the claims refer to these required co-acting essential conversion-coating-producing constituents as consists essentially of the (a) chromium trioxide constituent and the (b) lithium-cation-providing constituents. This language is used in the claims so as not to preclude the possibility that the claimed compositions may contain amounts of other materials, which generally would be significantly minor to that of the required co-acting essential conversion-coating-producing constituents.

The claimed compositions thus might contain one or more agents compatible with the chromium oxide and the lithium cation constituents, but for purposes other than that of the essential co-acting function of the chromium trioxide and the lithium cation in their primary provoking and provision of the complex or reaction, conversion corrosion-resistant coating. Such other substances are illustrated by the wetting agents, and buffering agents, whose purpose is other than conversion-coating-producing.

Neither a buffering agent nor a wetting agent is essential to the ordinary satisfactory operation of the conversion coating baths and method of the invention. Where it is desirable to operate at a selected pH fixed by some particular buffering agent compatible with the other ingredients of the bath, any such buffering agent can be used in suitable concentration in any of the above examples or others of different concentrations of the selected essential ingredients. Likewise, any compatible non-ionic or preferably anionic wetting agent can be used in place of any of them contained in any of the illustrative examples or in any of the earlier above-described possible modifications of any of them.

The devices or parts treated by immersion for a suitable time in any of the baths of the invention may be dried or baked under any conditions other than those disclosed in the specific examples. The drying and/or baking conditions are limited usually solely to being below any temperature, or any excessive time of exposure at a selected temperature, at which there could occur deleterious change in, or reaction of, the coating or any of its constituents (e.g. dye or pigment) with any of the contents of any drying gas or of the surrounding atmosphere. Simple laboratory drying or baking test can show whether any selected temperature is suitable or may be raised or should be lowered.

The final coatings produced by the treating baths and method of the invention are uniform, hard, ductile, adherent and non-peeling.

While the invention has been explained by detailed description of certain illustrative embodiments of it, it is understood that various substitutions and changes can be made in any of the illustrative embodiments within the scope of the appended claims which are intended also to cover equivalents of the specific embodiments.

What is claimed is:

1. A composition effective to produce a complex conversion coating providing enhanced corrosion-resistance on a metal member of the class consisting of ferrous and non-ferrous metals ordinarily subject to oxidation corrosion when exposed to an ambient oxidation-corrosion-inducing atmosphere, by immersing said metal for a sulficient time therefor in an aqueous solution of said composition which, as to its required co-acting essential conversion-coating-producing constituents, consists essentially of:

(a) chromium trioxide in the form of at least one member of the class consisting of chromic oxide, chromic acid, and a water-soluble dichromate; and

(b) a lithium-cation-providing constituent containing (i) a minor portion of a lithium halide other than its chloride, and (ii) a major portion of at least one water-soluble lithium salt Whose anion is compatible in aqueous medium with chromic acid which is more soluble than said lithium halide and also with dichromate anion; the content, by weight, of chromium from the chromium trioxide-contributing member being at least about equal to the total lithium cation content and which latter amounts to at least about two percent by weight.

2. A composition as claimed in claim 1, wherein the content of chromium from the chromium trioxide-contributing member is at least about one and one-half to about five hundred times the lithium cation content.

3. A composition as claimed in claim 2, wherein the content of chromium from the chromium trioxide-cont-ributing member is at least about seven times the lithium cation content.

4. A mixture composition effective to produce a complex conversion coating providing enhanced corrosionresistance on a metal member of the class consisting of ferrous and non-ferrous metals ordinarily subject to oxidation corrosion when exposed to an ambient oxidationcorrosion-inducing atmosphere, by immersing said metal member for a suflicient time therefor in an aqueous solution of said mixture in a suitable concentration in the water therefor; which mixture, as to its required co-acting essential conversion-coating-producing constituents, consists essentially of:

(a) chromium trioxide in the form of at least one member of the class consisting of chromic oxide, chromic acid, and a water-soluble dichromatic; and

(b) a lithium-cation-providing constituent containing (i) a minor portion of a lithium halide other than its chloride, and (ii) a major portion of at least one water-soluble lithium salt which is more soluble than said lithium halide and whose anion is compatible in aqueous medium with chromic acid and also with dichromate anion; said constituents (a) and (b) being so proportioned as to provide from about one to about five hundred parts of chromium from constituents (a) per part of lithium; and said mixture containing at least about 2.85 percent of lithium as cation.

5. A mixture as claimed in claim 4, wherein said constituents (i) and (ii) are proportioned to provide at least about seven parts of chromium per part of lithium.

6. An aqueous concentrate composition effective to produce a complex conversion coating providing enhanced corrosion-resistance on a metal member of the class consisting of ferrous and non-ferrous metals ordinarily subject to oxidation corrosion when exposed to an ambient oxidation-corrosion-inducing atmosphere, by immersing said metal for a sulficient time therefor in an aqueous solution of said concentrate which contains water to the extent of from about thirty to about sixty percent by weight thereof, and dissolved in the water as the required co-acting essential conversion-coating-producing constituents consisting essentially of (a) chromium trioxide in the form of at least one member of the class consisting of chromic oxide, chromic acid, and a water-soluble dichromate; and

(b) a lithium-cation-providing constituent containing (i) a minor portion of a lithium halide other than its chloride, and (ii) a major portion of at least one water-soluble lithium salt which is more soluble than said lithium halide and whose anion is compatible in aqueous medium with chromic acid and also with the dichromate anion; said co-acting constituents (a) and (b) being so proportioned as to provide from about one to about five hundred parts of chromium per part of lithium and which latter amounts to at least about two percent by weight.

7. An aqueous concentrate as claimed in claim 6, wherein said constituents (a) and (b) are proportioned to provide at least about seven parts of chromium per part of lithium.

8. A concentrate as claimed in claim 7, wherein said lithium halide minor portion (i) of the lithium-cationproviding constituent (b) is present therein in such proportion to the dillerent lithium compound (ii) thereof that the lithium content of said lithium halide portion (i) is from about 0.7% to about 55% of lithium in the lithium nitrate equivalent of said different lithium compound.

9. A concentrate as claimed in claim 8, which contains by weight about 47% of CrO about 2% of lithium fluoride, about 13% of lithium nitrate, and about 38% of water.

10. A concentrate as claimed in claim 8, which contains also from about 0.1 to about 0.5 percent of a perfluoro-octyl sulfonic acid wherein the octyl group is a member of the class consisting of open chain octyl, dimethylcyclohexyl, and ethylcyclohexyl.

11. A concentrate as claimed in claim 8, wherein the perfluoro-octyl group is a member of the class consisting of perfluoro (a) normal-octyl, (-b) 2,5-dimethylcyclohexyl, and (c) 4-ethylcyclohexyl.

12. A concentrate as claimed in claim 6, wherein by weight from about forty to about fifty-five parts of chromic acid, from about 0.3 to about 5.7 parts of lithium fluoride, and from about ten to about fifteen parts of lithium nitrate are dissolved in from about thirty' to about forty-nine parts of water.

13. A concentrate as claimed in claim 6, which includes in solution a water-soluble dyestuff compatible with the concentrate composition and stable at the operating conditions at which the aqueous conversion coating treating bath to be diluted from said concentrate is to be used, said dyest-ulf being in suflicient concentration in said concentrate to give the desired depth of its color in the finished conversion coating to be produced on the selected metal to be treated therefor in said bath.

14. An aqueous conversion coating bath etfective to produce a complex conversion coating providing enhanced corrosion resistance on a metal member of the class consisting of ferrous and non-ferrous metals ordinarily subject to oxidation corrosion when exposed to an ambient oxidation-corrosion-inducing atmosphere, by immersing said metal for a sufficient time therefor in said bath which has dissolved in its water .as the required co-acting essential conversion-coating-prod-ucing constituents thereof from about 1% to about 5% of the 1 1 aqueous concentrate of claim 6, so as to provide chromium content therefrom in the bath of at least about onehalf percent by weight, and lithium cation content of the bath of at least about one-tenth percent by weight.

15. A coating bath as claimed in claim 14, wherein the lithium cation content is at least three-tenths percent, and the chromium content from constituent (i) is at least about two percent.

16. The method of producing a complex conversion corrosion-resisting coating on clean surfaces of a metal member of the class of ferrous and non-ferrous metals oridnarily subject to oxidation corrosion on exposure to an ambient oxidation corrosion-inducing atmosphere, which method comprises (a) immersing said metal in an aqueous treating bath as claimed in claim 14 for a time sufficient for such desired coating to be produced on it at the temperature of the bath.

17. The method as claimed in claim 16, wherein the lithium halide minor portion (i) of the lithium-cationproviding constituent (b) is present therein in such proportion to the different lithium compound (ii) thereof that the lithium content of said lithium halide portion (i) is from about 0.7% to about 55% of the lithium in the lithium nitrate equivalent of said different lithium compound.

18. The method as claimed in claim 16, wherein the treating bath is maintained at from ambient temperature to about 100 F.

References Cited UNITED STATES PATENTS 2,868,679 1/1959 Pimbky 148-62 RALPH S. KENDALL, Primary Examiner. 

